Coatings for columbium and columbium base alloys

ABSTRACT

A method of protectively coating columbium and columbium-base substrates for service at temperatures above 2000* F comprising spray coating the substrate with a slurry consisting of a powdered mixture of silicon, aluminum and silver in a lacquer followed by diffusion treating the coated substrate in an inert atmosphere at a temperature in the range of 1900* F to 2300*F to form a layer of silicide intermetallic compounds adjacent the substrate and a layer of silver-silicon-aluminum alloy overlaying the first layer.

United States Patent 11 1 1111 3,753,763

Reznik Aug. 21, 1973 1 COATINGS FOR COLUMBIUM AND 3,293,068 12/1966Bradley at al 117/127 x Cowman BASE ALLOYS 2,763,919 9/1956 Kempe et al29/198 x inventor: Barry D. Reznik, Brooklyn, NY.

Assignee: The United States of America as represented by the AtomicEnergy Commission, Washington, D.C.

Filed: Jan. 24, 1964 Appl. No.: 340,112

References Cited UNITED STATES PATENTS 5/1967 .lefferys......'..........12/1966 Foldes et a1.

Primary Examiner-Carl D. Quarforth Assistant Examiner-Roger S. GaitherAttorney-Roland A. Anderson [57] ABSTRACT A method of protectivelycoating columbium and columbium-base substrates for service attemperatures above 2000 F comprising spray coating the substrate with aslurry consisting of a powdered mixture of silicon, aluminum and silverin a lacquer followed by diffusion treating the coated substrate in aninert atmosphere at a temperature in the range of 1900 F to 2300 F toform a layer of silicide intermetallic compounds adjacent the substrateand a layer of silver-silicon-aluminum alloy overlaying the first layer.

9 Claims, 3 Drawing F PATENTEI] MIB 2 1 I975 FIG. 2

TEMPERATURE INVENTOR.

BARRY D. REZNIK BY FIG COATINGS FOR COLUMBIUM AND COLUMBIUM BASE ALLOYSThis invention relates generally to the filed of metallurgy and isdirected in particular, to a new method of protectively coatingcolumbium and columbium alloys against oxidation at temperatures of2,000F and above, together with the article produced thereby.

Columbiumand its alloys possess a unique balance of properties that maketheir use particularly attractive for high temperature space and nuclearapplications. Where any of these applications require resistance tooxidation, however, protective coatings must be used, and theirdevelopment has been vigorously pursued for a number of years.

Heretofore protective coatings for columbium and its alloys have beensuccessfully developed which will provide short time protection of fromminutes to a maximum of about 100 hours at temperatures up to about2,600F. The most successful of these coatings have been obtained by packcoating with a mixture of titanium-chromium and silicon, or by slurrycoating with a tin-aluminum alloy.

The foregoing coating methods, while suitable for some applications, areunsatisfactory from certain points of view. For example, thetin-aluminum slurry coatings of the prior art can not be relied upon forprolonged protection at 2,000F since it has been found that at thistemperature the aluminum in the coating diffuses into the substrate andcombines to form Cb,Al. This gradually removes aluminum from the coatingand leads to failure in the course of time at 2,000F. At highertemperatures, such as 2,500F and above, the tin-aluminum base coatingshave no useful life on columbium or columbium base alloys.

The Ti-Cr-Si coating, although effective in its ability to protectcolumbium substrates from oxidation, is both expensive and timeconsuming to apply since it requires multicycle high-temperature vacuumheating treatments. Additionally, the high temperatures employed in thecoating process has an adverse effect on the mechanical properties ofthe substrate.

It is the object of this invention to provide a new method of coatingcolumbium and columbium base alloys so as to protect the columbiumagainst oxidation at temperatures of approximately 2,000F for at least aperiod of hundreds of hours. It is a further object to provide a coatingthat is readily and economically applied to the columbium or columbiumbase alloy and which can be applied without adversely affecting themechanical properties of the columbium.

The foregoing objects are achieved by coating the columbium with alloysof silver and silicon or of silver, silicon and aluminum. Thereafter thecoated article is given a difl'usion treatment in a vacuum or in argonto form a coating consisting of a layer of silicide intermetalliccompounds adjacent the substrate, over which is a layer of thesilver-silicon base alloy.

The invention may be more completely understood by reference to thefigures, wherein FIG. 1 is a photomicrographic (300x) of columbiumcoated with an alloy consisting of Ag-lO Si 1 Al in accordance with thisinvention.

FIG. 2 is a photomicrograph (300x) of a specimen as in FIG. 1 afterexposure for 100 hours at 2,500F., and

FIG. 3 is a graph comparing the oxidation life of an Ag-lS Si-l Alcoating with a Ti-Cr-Si coating on columbium under cylic conditions attemperatures between 2,600F and 3,000F.

The coatings of the present invention are prepared by first suitablycleaning the substrate in any suitable manner, such as by blasting withangular steel grit.

Next, the substrate is coatedby cold spraying waits" slurry made up of amixture of fine (minus 300 mesh) silver, silicon and/or aluminum metalpowders with an equal volume of lacquer. After air drying, the sample isgiven a difiusion treatment in a vacuum or in an argon atmosphere forperiods of from 10 to 15 minutes at temperatures in the range of l,900Fto 2,500F., to form a coating consisting of a layer of silicideintermetallic compounds, presumably Cb Si adjacent the substrate.Overlying the silicide layer is an outer layer of a Ag-Si-Al alloy, asmay be seen by inspection of FIG. 1.

After oxidation for 100 hours at 2,560 a specimen as in FIG. I, retainsa protective coating, as shown in FIG. 2. It is important to note thatthe inward diffusion of the coating into the substrate is slow;amounting to only 1.25 mil penetration after 100 hours at 2,500F.Metallographic studies of samples oxidized at 2,000? show that the rateof conversion of CbSi, to Cb Si, is slow and would require 6,000 hoursat that temperature to convert a coating containing a thickness of 1 milCbSi, to Cb -,Si,,. It is therefore apparent that this coating, appliedin suitable thickness can achieve the desired goal of protection for thedesired period of 2,000F. i

It has been observed that no one coating is equally protective to allcolumbium alloys, and that with the development of each new alloy onemust either reevaluate or modify an existing coating or develop anentirely new coating in order to provide the required oxidationprotection. Because of this unpredictability in the behavior ofcolumbium and its alloys, experiments were conducted with columbium, andtwo columbiumbase alloys identified respectively as X-l l0 (Cb-10W- lZr-0.6C) and 8-66 (Cb-5Mo-5V-l Zr). The latter two alloys are ofinterest because of their superior mechanical properties as compared,for example, with an alloy such as Cb-l Zr, and coupled with their goodfabricability. Results of preliminary screening tests are given in thefollowing Table I. In each instance, the diffusion step was repeated toincrease the thickness of the coating:

TABLE I Oxidation Protectlveness oi Ag-St Base Slurry Coating DiffuslonOxidation life (hours) Base Coating treat alloy composition ment 1,100F. 2,000 F. 2,500 F.

321-474 Cb Ag-25Si A 100 so 19 Cb Ag-17Si A 175 260 100 Cb Ag-IOSl B 110196 83 Ch Ag-lOSl-lAl A 200 300 105 Cb Ag-10Sl-5Cr-1Al A 100 200 30 40B102-46-19 Cb Ag-10Sl-5Cr-5'Il A 100 200 22 27 TABLE I OxidationProtectiveness oi Ag-Sl Base Slurry Coating Diffusion Oxidation life(hours) Base Coating troat- Sample No. alloy composition ment l,100 F.2,000 F. 2,500 F.

11102-32-1 Cl) Ag-15Sl-5Cr A 470 800 150 150 70 D2 13102-31-11.Ag-15Si-1Al A 150 500 100 310 101 B'Z 52-L. Ag-IOSi-lAl I! 610 700+ 100280 40 55 13102-5241 Ag-IOSl-iAl B 400 680 60 98 55 74 3102-52-21Ag10Si-1Al B 350 350 122 122 55 70 13102-54-1 Ag-lilSi-l Al C 60 60 6030 40 74 13102-60-1 Ag-Si-1Al B 1. ND 264 255 50 60 B102-60-6Ag-lSSi-ZiAl B ND 315+ 315+ 50 50 B10260l1 Cb Ag-15Sl-5Al B ND 315+ 315+50 50 Nora-Diffusion treatment:

A=double diffusion 10 minutes at 2.300 F. in argon, B=double diffusion15 minutes at 2,300 F. in argon, C=double diffusion 15 minutes at l,900F. in vacuum. ND =Not determined.

' As diffusion treatments in vacuum result in substantial losses of Agby volatilization, it is deemed preferably to employ an argon atmosphereat 1 atmosphere pressure to minimize such losses.

The effectiveness of the Ag-Si-Al coating on pure columbium under cyclicoxidation conditions at temperatures of 2,600F and above is shown in H6.3 which shows that this coating has twice the effective life at 3,000Fas the best known coating, Ti-Cr-Si.

Subsequent tests illustrating the effectiveness of the Ag-Si-Al coatingson B-66 and X-l 10 alloys are summarized in the following Table 2. Thefindings listed in the following Table 2 also show the value ofpreoxidation of the coated sample in increasing oxidation resistance attemperatures of l,l00F and 1,800F. in the examples given in the table,the preoxidation step was conducted in air at about 2,500F for a periodof 2 hours and resulted in a substantial increase in low temalso permitsthe oxide film to grow on the liquid, thereby eliminating stressesbetween the surface oxides and the intermetallic compounds.

it has been observed that the best coatings for service at temperaturesof 2,500F and above are the Ag-IS Si- [Al coatings, as illustrated inFIG. 3. However, for use at 2,000F coatings of Ag-lS Si-7.5 Al and Ag-15Si-8 Al give the best results. It is believed that at the lowertemperature greater amounts of aluminum are useful in preventing loss ofAg by volatilization. Al,0, forms preferentially on oxidation at 2,000F.As loss of Ag through M 05 is slow, the sample initially gains weight asthe Al,0, forms. As oxidation progresses, Si0, also forms and grows. Atthe same time the concentration of Al,0, decreases to a point where itbecomes permeable to the Ag. Thereafter, loss of Ag exceeds the rate ofoxidation of the sample, and the latter loses weight.

perature resistance of the samples. Hence, the higher the aluminumconcentration the TABLE 2 Samples preoxidized Oxidation life, hours 2hrs., 2,500 F. Base Coating compo- Sample alloy sition(w/o) 1,100 F.1,800 F. 2,000 F. 1,100 F. 1,800F

B102821 (1b Ag-15Si-1A1 200 417 200 200 590 700 ND 340 B-102-829..-Ag-15Si-3Al 200 66 200 200 00s 050 ND 300 3402-82-17.. Ag-15Si-5Al 200200 270 300 see 1,000 ND ND B-l02-86-1 Ag-15Si1Al 245 630 585 800 870800 13-102-86-9 Ag-15Si-3Al 3-102-86-1 Ag15Si-5Al B-102-88-1 Ag-ISBi-lAlB-102-ss-0... Ag-15Si-3Al B-102-ss-17.. Ag-15Si-5Al 13-271-0-10...Ag15Sl7.5Al B-277-s-1.... Ag-l58i-5Al B-277-s-4.-.. Ag-20sr-7ArB-277-s-7.... Ag-15Bi-7.5Al B-277-s-11... Ag-25Si-8Al n-z17-7-1....Arr-15Bi-5Al B-277-1-4. Arr-208i-7Al B-277-7-7 Ag-sasi-sAl 13-277-0-Ag-rssl-sAl Norm-ND =Not Determined.

Useful coatings may be obtained in the range of 3 to 30 w/o Si and l to10 w/o Al, the balance being Ag. However, it is to be noted from theresults tabulated in the foregoing Table 2, that for service at 2,000F,the longest oxidation life was achieved with a slurry consisting ofAg-l5 Si-7.5Al.

It has been found that the aluminum is useful in improving the lowtemperature and thermal cycle capabilities of this coating. In contrast,additions of Cr and Ti have been shown to be detrimental.

While the reason for the effectiveness of the present coating is notcompletely understood, it is apparent that liquid silver-silicon balealloy acts as a reservoir for repairing defects in the surface oxidefilm, and it longer it takes for losses of Ag to assume significantproportions. At temperatures of 2,500F and above, however, a differentmechanism appears to be operative since it has been observed thatconcentrations in excess of l w/o Al actually decrease the oxidationlife of the sample. It is believed that this decrease in oxidation liferesults from the formation of lower melting point oxidation products atthis temperature.

From the foregoing it will be seen that useful coatings for columbiumand columbium base alloys have been produced which will protect the basemetal against oxidation at temperatures of 2,000F and above for usefulperiods of time.

I claim:

1. The method of protectively coating columbium and columbium-basesubstrates for service at temperatures of 2,000F and above thatconsistsof the steps of:

a. Cleaning the substrate;

b. Preparing a powdered material consisting of silver,

silicon and aluminum powders in the following proportions by weight: 3to 30 percent silicon; l to percent aluminum and the balance silver; e.Preparing a slurry by suspending said material in a lacquer;

d. Spray coating the substrate with the slurry; and

e. Diffusion treating the coated substrate in an inert atmosphere at atemperature in the range of approximately l,900F to 2,300F to form alayer of silicide intermetallic compounds adjacent the substrate and alayer of silver-silicon-aluminuin alloy overlying the said silicideintermetallic compounds.

2. The method ot' claim l wherein the pOWdeEedma terial consists of percent by weight silicon, 8 per cent by weight aluminum and the balancesilver.

3. The method of claim I wherein the coated sample is preoxidized in airat atemperature of approximately 2,500F following diffusion treatment toimprove oxidation resistance of the sample.

4. The method of claim 1 wherein the columbiumbase alloy has thefollowing composition by weight per cent:

Cb-S MO'SV- lZr.

5. The method of claim 1 wherein the columbiumbase alloy has thefollowing composition by weight percent: g Cb 10W l Zr 0.6C.

6. A columbium substrate having an oxidation protective coating thereonsaid coating consisting of a sili cide intermetallic layer on thesubstrate and a silversilicon-aluminum alloy layer on the saidintermetallic layer.

7. The article of claim 6 wherein the substrate con-; sists of acolumbium base alloy having the followingl 9. The article of claim 6wherein the silicide intermetallic layer has the approximatecomposition: Cb Si,.

1. The method of protectively coating columbium and columbium-basesubstrates for service at temperatures of 2,000*F and above thatconsists of the steps of: a. Cleaning the substrate; b. Preparing apowdered material consisting of silver, silicon and aluminum powders inthe following proportions by weight: 3 to 30 percent silicon; 1 to 10percent aluminum and the balance silver; c. Preparing a slurry bysuspending said material in a lacquer; d. Spray coating the substratewith the slurry; and e. Diffusion treating the coated substrate in aninert atmosphere at a temperature in the range of approximately 1, 900*Fto 2,300*F to form a layer of silicide intermetallic compounds adjacentthe substrate and a layer of silver-silicon-aluminum alloy overlying thesaid silicide intermetallic compounds.
 2. The method of claim 1 whereinthe powdered material consists of 15 per cent by weight silicon, 8 percent by weight aluminum and the balance silver.
 3. The method of claim 1wherein the coated sample is preoxidized in air at a temperature ofapproximately 2,500*F following diffusion treatment to improve oxidationresistance of the sample.
 4. The method of claim 1 wherein thecolumbium-base alloy has the following composition by weight per cent:Cb - 5 Mo - 5V -1Zr.
 5. The method of claim 1 wherein the columbium-basealloy has the following composition by weight percent: Cb - 10W - 1 Zr-0.6C.
 6. A columbium substrate having an oxidation protective coatingthereon said coating consisting of a silicide intermetallic layer on thesubstrate and a silver-silicon-aluminum alloy layer on the saidintermetallic layer.
 7. The article of claim 6 wherein the substrateconsists of a columbium base alloy having the following composition byweight: Cb - 5 Mo - 5 V - 1 Zr.
 8. The article of claim 6 wherein thesubstrate consists of a columbium-base alloy having the followingcomposition by weight: Cb - 10W - 1 Zr - 0.6C.
 9. The article of claim 6wherein the silicide intermetallic layer has the approximatecomposition: Cb Si2.